Significant generalized osteopenia develops during childhood or adolescence in some 40% of JRA patients. Attainment of life long peak bone mass occurs during adolescence for females and has been shown to be decreased in many JRA patients. There are neither accepted treatments nor controlled trials addressing osteopenia in JRA patients. Observational studies in JRA patients document high prevalence of suboptimal dietary intake of calcium (Ca) and vitamin D, significantly lower gastrointestinal Ca absorption and low bone formation rate. In JRA patients, daily oral supplementation of 1000 mg of Ca (Ca carbonate) and 400 I.U. of vitamin D for 24 months will result in at least a 10% greater increase in total body bone mineral density (BMD) measured by dual energy x-ray absorptiometry (DXA) compared to treatment with placebo and 400 I.U. vitamin D and this increased BMD will persist for at least 18 months after cessation of the trial. In addition, this project will determine the effect of Ca supplementation on bone physiology by measuring serum and urinary bone related minerals (ca, phosphorus), hormones (parathyroid and vitamin Ds), bone formation markers (urinary Ca/creatininine and pyridonoline crosslinks). The effects of variants of the vitamin D receptor allele on bone physiology and response to Ca supplementation will also be determined.